Test downsampling blits in GL_LINEAR filter mode.

Since downsampling blits require the source and destination rectangles
to be exactly the same size, GL_LINEAR and GL_NEAREST filtering should
be equivalent.  So just add an option to the "accuracy" test that
causes it to do a GL_LINEAR downsampling blit.
Test upsampling blits in GL_LINEAR filter mode.

Since upsampling blits require the source and destination rectangles
to be exactly the same size, GL_LINEAR and GL_NEAREST filtering should
be equivalent.  So just add an option to the "upsample" test that
causes it to do a GL_LINEAR upsampling blit.
Test MSAA-to-MSAA blits in GL_LINEAR filter mode.

Since MSAA-to-MSAA blits require the source and destination rectangles
to be exactly the same size, GL_LINEAR and GL_NEAREST filtering should
be equivalent.  So just add an option to the "multisample-blit" test
that causes it to do a GL_LINEAR blit.